The present invention relates generally to circuitry for protecting a ballast from short circuit currents.
An electronic ballast typically provides a rectified line voltage through a Power Factor Correction (PFC) circuit to an inverter such as a half-bridge circuit, and the inverter then converts the direct voltage into a high-frequency alternating voltage for driving a fluorescent gas discharge lamp. To maximize the effective life (which is reduced with each cold start) of a fluorescent lamp, it is known to provide a xe2x80x9crapid-startxe2x80x9d preheating circuit within the electronic ballast. The heater windings of the preheating circuit warm a filament within the fluorescent lamp (normally by providing a controlled current or a controlled voltage) before the lamp is switched on. As in low-frequency hybrid ballast technologies, it is also known to provide xe2x80x9cmodified rapid-startxe2x80x9d circuitry that disconnects cathode heater windings after the lamp ignites, thereby reducing the energy consumed per lamp during steady-state conditions.
The proper wiring configuration between the input power line and the lamp driven by a ballast is determined by the type of lamp that the ballast will drive. If the fixture is improperly wired a short circuit of the lamp terminals will occur, which leads to damage within the ballast and/or the lamp manifested by reduced lamp life, failure of the lamp to ignite, and/or premature failure of the ballast.
Known short circuit proofing approaches are relatively costly and typically rely on parameters such as switching frequency or component tolerances, thereby making the design rigid. For example, a frequency dependent filament heating circuit may employ filament voltage oscillators and a detector circuit for each filament to control filament voltage during an interrupt or fault condition. In this circuit arrangement, the filament voltage oscillators are separate from and operate independently of an arc voltage oscillator. Furthermore, any short circuit protection scheme for a universal ballast (i.e., a ballast that can drive different lamp types) that uses frequency-dependent components must be designed to achieve a stable and accurately controlled filament voltage. Such a design is complicated by the fact that filament impedances typically vary according to lamp type. Thus, such an approach is disadvantageous due at least in part to the cost of the more rigid development process.
Over-current protection can also be provided using a silicon controlled rectifier and a zener diode. When the output current increases to the maximum value and the voltage across the zener diode reaches its breakdown voltage, the zener diode charges a capacitor that eventually fires the rectifier which then disables the oscillation control circuit. Thus, because this approach also depends on the tolerances of both the zener diode and the rectifier it is relatively costly to design.
There is a need for an improved and less expensive configuration of a preheating circuit that includes protection against short circuits caused by improper wiring and that is independent of switching frequency and component tolerances.
The circuit of the present invention fulfills the need described above by providing a preheating circuit that warms lamp electrodes in a relatively short time, produces an accurate filament voltage, and that operates independently of switching frequency of the inverter, duty-cycle of the switch(es) in the inverter, filament resistance or other similar parameters. Furthermore, the circuit is short-circuit proof, cost-effective, and compact. The preheating circuit includes a sense resistor to detect and prevent damage caused by short circuit conditions. In the event that a short circuit occurs, the voltage across the sense resistor acts as a control signal that switches the preheating circuit off. The exemplary short-circuit protection circuit configuration of the present invention has the advantage of being simple to design because it is practically independent of the switching frequency and tolerances of the ballast components.
Briefly, the present invention provides a preheating circuit as a rapid-start mechanism to preheat the electrodes of a fluorescent lamp before the required voltage for striking an arc between the electrodes is applied. An exemplary circuit according to the present invention heats the electrodes through a transformer, comprising a primary winding and two secondary windings, and a small sensing resistor connected in series with the primary winding and fed by a voltage source such as a half-bridge circuit. The sensing resistor functions such that a short circuit will cause the voltage across the resistor to exceed a predetermined level which triggers switching the voltage source off. This electrode heating circuit can be used in a multi-lamp ballast, as the circuit enables such a ballast to withstand individual short-circuits that can occur randomly between lamps driven by the ballast.
A further exemplary embodiment of the present invention is a ballast circuit that is connected to two fluorescent lamps, each lamp having two ends with a filament at each end, and each filament connected to an electrode. The ballast circuit includes a half-bridge circuit that supplies voltage to a preheating circuit which preheats the electrodes of each lamp driven by the ballast. The preheating circuit includes a filament heating transformer that has a primary winding and three secondary windings. The primary winding is connected in series with a relatively large capacitor that performs DC-blocking (i.e., removes DC voltage bias from the supply voltage). The ballast circuit also includes a short-circuit protection circuit that detects elevated currents in the primary winding of the preheating circuit, a condition which indicates the presence of a short-circuit of at least one secondary winding of the preheating circuit. The short-circuit protection circuit utilizes a sensing resistor that is coupled to the preheating circuit to sense the current flowing through the primary winding preheating circuit. The sensed current is converted into a rectified average DC voltage (which is used as a control signal) by a control signal circuit that is connected across the sensing resistor. The control signal circuit is the combination of a rectifying diode and a low-pass filter that ensures that transitory currents (e.g., inrush and start-up currents) do not trigger the disengagement of the preheating circuit. Rather, a control means (for example, a MOSFET) connected between the preheating circuit and the sensing resistor operates to disengage the supply voltage if non-transitory overcurrent conditions occur such that the control signal exceeds the value of a preset trigger level. A clamping diode prevents the voltage across the MOSFET from exceeding the bus voltage.
Filament sensing is performed by some ballasts. Accordingly, in an alternative embodiment, the ballast circuit includes a capacitor and a circuit part for generating a DC current through one or more lamp filaments and for detecting that DC-current. The capacitor blocks a path of the DC current that does not comprise the lamp filaments but one of the secondary windings of the filament heating transformer in the event that the filament sensing circuit determines that a lamp filament is not present. The DC current path is interrupted if a filament is missing or otherwise cannot conduct. This interruption is detected and the ballast circuit can for instance be switched off.
An advantage of the preheating circuit according to this exemplary ballast circuit is that the filament heating transformer can be relatively small in size because the filament heating transformer needs only to carry a current during startup of the ballast (while the lamp electrodes are heated), and can be switched off during normal operation.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become more apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.